How motives does Steinbeck suggest for her behavior?

Are you referring to Curley's wife?

yes, Steinbeck is Curley's wife

Curly's wife is flirtatious throughout the book. Basically whenever she shows up she is flirtatious because this is all she has. Curly's wife is powerless in a masculine world, Steinbeck doesn't even give her a name; she is simply Curley's property. Check out this quote where she uses her femininity for attention,

"Oh!" She put her hands behind her back and leaned against the door frame so that her body was thrown forward. "You’re the new fellas that just come, ain’t ya?" (chapter 2, pg 145)

Note: my page numbers probably don't match your but the chapter will.